Such a digital transmission system is, for example, a synchronous transmission system according to the SDH/SONET standard. In such a digital transmission system, individual network elements are interconnected by different transmission media (e.g., copper cables, optical fiber waveguides, or radio links).
The connection between a network element and a transmission medium is provided by interface units ("network node interfaces"), which are known, for example, from ITU-T Recommendations G.703 and G.957. Recommendation G.703 specifies electrical characteristics of such interface units, and Recommendation G.957 specifies optical characteristics of such units. In an interface unit connected to an optical fiber waveguide, incoming optical signals, for example, are converted to electric signals by an optical-to-electrical transducer. Consequently, outgoing electrical signals are converted to optical signals by an electrical-to-optical transducer. During such a conversion, the clock frequency of the respective signal does not change.
In each network element, which is a cross-connect, for example, an internal clock frequency serves as a clock reference. Deviations of the clock frequency of an incoming signal from this internal clock frequency gives information about the synchronization status of the transmission system. An overview of the subject of synchronization is given in an article by W. E. Powell et al, "Synchronization and Timing of SDH Networks", Electrical Communication (Alcatel), 4th Quarter 1993, pages 349 to 358.
To monitor the synchronization status, it is known, (Report on ETSI-TM3 Meeting in BristoL, October 1992, page 205) to monitor the pointer activity. Monitoring the synchronization status with the aid of the pointer activity is required in BeLLcore
TA-NWT-000 436. Such monitoring is disadvantageous in that because of an unstandardized hysteresis associated with the pointer processor, the information derived from the pointer activity permits only qualified statements about the synchronization status.